Seems like all prop manufacturers recommend balancing, so I do it using a magnetic type like CGR described. Usually they aren't very far out of balance. I never try to shave or sand the prop however. I always spray clear acrylic on the light side instead. The Master Airscrew website says that if a piece of masking tape 1" x 1" or less in size applied to the tip of the light side makes it balance or makes it the heavy side then you are within tolerance.

Rufcut